Science

Absolute zero is the zero point of thermodynamic temperature, equal to 0 kelvin or −273.15 °C, at which thermal excitation reaches its limiting minimum.

Absorption is the uptake of a substance into the bulk of another material, often used to separate gases by dissolving them in liquids.

Action is a scalar functional of a system’s history whose stationary values determine classical dynamics and whose phase governs quantum path integrals.

Activated carbon is a highly porous carbon material used to adsorb substances in liquid and gas purification, industrial processing, and selected medical applications.

Adaptation is evolutionary adjustment to an environment, or a trait shaped by selection; more broadly, it denotes adjustment to changing conditions.

An adiabatic process is a thermodynamic change during which no heat is transferred between a system and its surroundings.

Adsorption is the accumulation of substances at an interface through physical interactions or chemical bonding, enabling separation, purification, and surface characterization.

An adsorption isotherm describes how the equilibrium amount adsorbed varies with pressure or concentration at a fixed temperature.

Alfred Russel Wallace was a British naturalist who independently developed natural selection and helped establish evolutionary biogeography.

An allele is an alternative DNA sequence at a particular genomic location, contributing to inherited variation within and between populations.

Alpha decay is radioactive transformation in which an unstable nucleus emits a helium-4 nucleus, reducing its atomic number by two and mass number by four.

The alpha helix is a common protein secondary structure consisting of a coiled polypeptide backbone stabilized by regularly spaced hydrogen bonds.

Alternative splicing produces different RNA transcripts from one gene by varying which segments of a precursor RNA are retained and joined.

Ammonia is a nitrogen–hydrogen compound essential to fertilizer production, industrial chemistry, and the nitrogen cycle.

The ampere is the SI base unit of electric current, defined by fixing the elementary charge at exactly 1.602176634 × 10⁻¹⁹ coulombs.

Analytical chemistry develops and applies methods for identifying substances, measuring their amounts, and establishing the reliability of chemical measurements.

An experimental technique that measures photoelectron energies and emission angles to investigate the momentum-dependent electronic structure of materials.

The ångström is a non-SI unit of length equal to 10⁻¹⁰ metre, used chiefly for atomic dimensions, molecular structures, and wavelengths.

Angular momentum measures rotational motion and is conserved when the net external torque vanishes; in quantum mechanics, it includes orbital and intrinsic spin contributions.

The anomalous magnetic moment measures a particle’s deviation from the Dirac magnetic-moment prediction and provides a precision test of quantum interactions.

Antibiotic resistance is the ability of bacteria to withstand drugs intended to inhibit or kill them, complicating infection treatment and control.

Antonie van Leeuwenhoek was a Dutch microscopist whose observations of bacteria and other microscopic organisms helped establish microbiology.

Archaea are microorganisms distinguished from bacteria by their evolutionary history, membrane chemistry, and cellular machinery, with major roles in global nutrient cycles.

Astrophysics applies physical laws to explain the properties, behavior, origins, and evolution of celestial objects and the universe.

Asymmetric catalysis uses catalysts to favor the formation of one stereoisomer, enabling selective synthesis of chiral compounds.

Asymptotic freedom is the weakening of an interaction at increasing energy scales, a defining property of quantum chromodynamics.

ATLAS is a general-purpose particle-physics experiment at CERN’s Large Hadron Collider, designed to investigate fundamental particles and interactions.

An atom is the smallest unit of a chemical element. It has a dense nucleus of protons and neutrons surrounded by electrons.

The atomic nucleus is the compact, positively charged center of an atom, containing protons and usually neutrons and accounting for nearly all its mass.

Atomic number is the number of protons in an atomic nucleus, defining an element’s identity and its position in the periodic table.

An atomic orbital is a one-electron wavefunction describing the spatial quantum state of an electron in an atom.

ATP synthase is a membrane-associated rotary enzyme that couples ion flow to the production of ATP, the principal energy currency of cells.

The Avogadro constant is the exact proportionality constant relating the number of specified entities to amount of substance, with a value of 6.02214076 × 10²³ per mole.

Bacteria are prokaryotic organisms forming a major domain of life, with diverse cellular structures, metabolisms, ecological roles, and relationships with other organisms.

A bacteriophage is a virus that infects bacteria, influencing microbial populations, genetic exchange, and biological research.

A band gap is an energy interval without allowed electronic states, central to the electrical and optical properties of semiconductors and insulators.

Barbara McClintock was an American cytogeneticist who discovered mobile genetic elements in maize and received the 1983 Nobel Prize in Physiology or Medicine.

A baryon is a composite particle with baryon number one, usually containing three valence quarks; protons and neutrons are the best-known examples.

BCS theory explains conventional superconductivity through electron pairing and the formation of a coherent quantum state.

A Bell state is one of four maximally entangled two-qubit states that form an orthonormal basis and serve as fundamental resources in quantum information.

Beta decay is a weak-interaction process that converts neutrons into protons or protons into neutrons, accompanied by leptons.

A beta sheet is a protein secondary structure formed by extended polypeptide strands joined through backbone hydrogen bonds.

The Big Bang model describes the universe’s expansion and cooling from an early hot, dense state, supported by cosmic expansion, background radiation, and primordial element abundances.

Binary fission is a form of asexual reproduction in which one cell divides into two daughter cells, commonly occurring in bacteria, archaea, and some unicellular eukaryotes.

Biochemistry studies the molecules, chemical reactions, and regulatory processes that underpin the structure and functioning of living organisms.

Biology is the natural science of life. It studies the structure, function, growth, origin, evolution and distribution of living organisms.

A biopsy is the removal of cells or tissue from a living person for laboratory examination to diagnose or characterize disease.

A black hole is a region of spacetime bounded by an event horizon, beyond which nothing can escape to the outside universe.

Blackbody radiation is the universal thermal electromagnetic radiation spectrum of an ideal absorber, determined solely by its absolute temperature.

The Boltzmann constant connects temperature with microscopic energy and statistical entropy, and its exact value defines the kelvin.

The Boltzmann distribution assigns equilibrium probabilities to physical states according to their energies and the temperature of a thermal reservoir.

The Born rule connects quantum states to measurement outcomes by assigning probabilities through squared amplitudes or equivalent operator formulas.

A quantum state of bosonic matter in which a macroscopic fraction of particles occupies a single quantum state, producing collective behavior and coherence.

Bose–Einstein statistics describes the quantum counting and equilibrium occupation of states by identical bosons, allowing multiple particles to occupy the same state.

A boson is a quantum particle with symmetric exchange statistics, allowing identical particles to occupy the same quantum state.

A geometric relation connecting wavelength, crystal-plane spacing, and the angles at which constructive diffraction occurs.

Brunauer–Emmett–Teller theory models multilayer gas adsorption and provides a widely used method for estimating the specific surface area of solids.

Calibration establishes relationships between reference values and instrument indications, including their uncertainties, so that indications can be converted into measurement results.

Calorimetry measures heat exchanged during chemical reactions and physical processes, enabling determination of energy changes, heat capacities, and thermal transitions.

Cancer is a group of diseases characterized by uncontrolled growth of abnormal cells, invasion of surrounding tissues, and potential spread to distant organs.